You can equip up to two skulls per mission. When a mission is completed with skulls activated, you will receive a credit bonus. Having two skulls equipped increases the credit bonus. Skulls will handicap you with the following effects:

Hollow: Your Spartan only has a shield for protection.

Pacifist: Your Spartan's armor and shields deplete when you fire a weapon.

Airstrike (1,000 credits): Several explosives that last for 2.5 seconds.

Steam achievements

Successfully complete the indicated task to unlock the corresponding achievement. To view your achievements and stats in Steam, select "Community", "My profile", "View all my games", then the game and view stats.